The purpose of a homing assembly is to allow the control system of an automatic sewing machine to find a given "home" position from any random starting point within the range of motion of the physical system. To be effective, a workholder positioning mechanism must be operative throughout the full range of travel of the workholder. Like the present invention, U.S. Pat. No. 4,002,128 granted Jan. 11, 1967 to F. F. Yanikoski concerns an automatic sewing machine which employs stepper motors and a homing assembly having a single sensor assembly for positioning the workholder. In the Yanikoski device, one revolution of the stepping motor drive shaft was effective to move the workholder over its full travel approximating 41/2" by 21/2". With this limited range of movement, coupled with less than one full revolution of the stepping motor drive shaft, a homing assembly using a single sensor assembly was effective to accomplish the desired result. It will be understood that a homing assembly driven from the stepping motor and which includes a revoluable indicator disc must be sized and driven such that the disc's excursion or arcuate travel must be less than 360.degree. for the full excursion of the mechanical system. An arcuate path of travel exceeding 360.degree. would result in redundancy and ultimately inaccurate readings. In addition, the homing sensor's band of uncertainty must be considered in the design of a homing assembly.
The homing assembly shown in the above-identified patent does not lend itself to the present invention for the following reasons. With the present application of the invention, the full range of clamp movement approximates 10".times.10". Thus, the single indicator disc concept used in the Yanikoski device is not practical with the instant invention. Because the clamp excursion of the present invention is so much larger than that used in Yanikoski, the diameter of the indicator disc would have to be significantly increased to not exceed its allowable 360 degree path of arcuate travel. The resultant size of such an indicator disc is impractical. To reduce the arcuate travel of a single disc indicator would require sacrificing the positional accurancy of the workholder relative the needle. Any reduction in the arcuate travel of a homing sensor magnifies its inaccuracy in the sensor's band of uncertainty, a result which must be avoided. Therefore, the homing assembly shown in the Yanikoski patent would not readily lend itself to incorporation within the automatic sewing machine here under consideration.